World PvP and how it stands/standed.
World PvP isn't meant to be balanced, fair nor honorable. It's meant to be brutal, relentless, deadly.. and it's the survival of the fittest. It's a fight where everything goes and you use anything you can to survive and win. It's War.. a World of Warcraft where only the best survive!

Well, some people saying necromancy but I'm more for "hat magic" than necromancy. The lore we have about Frosty is simply that the hat brings him to life -- not that a dead person's soul is trapped in Frosty, or that Frosty is undead. Demonic possession is another good way to go, with the hat as a medium. This would trump being a Frost elemental (As you could postulate that all elementals are demon-possessed). Nature is also a very good way of doing it, but it'd have to be through the hat (given the lore). So what we have to fall back on is something that can enchant the hat to bring Frosty, a normal snowman, to life to be Frosty the Snowman.

I guess after all of that... I've love to say demonic possession as being the most likely. But it could also be that the hat is a very powerful and old magical artifact wherein the owner's magic is either tied into it somehow (immortality stuff), or he himself is, and when the hat is placed on the snowman... The owner of the hat uses the body to come to life.

'Tis Necromancy! His haunted husk is an unholy blight upon the Earth! He must be purged with holy flames and the ground upon which he melts must be salted so as to ensure that he is banished forever to the shadowy abyss that hath spawned him!

Pure hatred. With a heart as cold as winter's chill, his physical body is the manifestation of evil itself. He is a creature of wrath, destruction, and bloodlust. Frosty the Snowman is simply the disembodied soul of a disgruntled and disturbed telemarketer who died in the workplace after his superior refused to turn on the heater during the frigid month of December. Overworked, underpaid, and marginalized by every man and woman who wouldn't let him finish his sales pitch before hanging up, he now has a bone to pick with us all. Heaven help us.

Keep in mind that Frosty does not merely inhabit a single body. The children have to rebuild him every year.

Seems like chronomancy to me. To achieve immortality, a very powerful wizard transferred his consciousness into the hat to exist in a state outside the realms of space and time. When the hat makes contact with an inanimate, humanoid object, it recreates the link between the physical world and Frosty's dimension. To him, physical reality is meaningless. He exists simultaneously in every body the children have ever placed him in, spanning countless ages as though they were all happening at once.

As to why he says "Happy Birthday!" every time he is reanimated, the answer is quite simple. When Frosty cast his spell of chronokenesis, the effect was initially delayed. He then attended a neighbor's surprise party. As the neighbor came in, everyone yelled "HAPPY BIRTHDAY!" except Frosty, who was longer in the mortal plane. The spell took effect at the exact moment between his mind giving the command to say "Happy birthday", but before his body could act on that command. And because Frosty exists in every life he's ever lived all at once, at that moment he sent the command to each of his reincarnations simultaneously to say "happy birthday".